Development of a Finite Element Analysis Methodology for the Propagation of Delaminations in Composite Structures

Analysing the collapse of skin-stiffened structures requires capturing the critical phenomenon of skin-stiffener separation, which can be considered analogous to interlaminar cracking. This paper presents the development of a numerical approach for simulating the propagation of interlaminar cracks in composite structures. A degradation methodology was applied in MSC.Marc that involved modelling the structure with shell layers connected by user-defined multiple point constraints (MPCs). User subroutines were written that apply the Virtual Crack Closure Technique (VCCT) to determine the onset of crack growth, and modify the properties of the user-defined MPCs to simulate crack propagation. Methodologies for the release of failing MPCs are presented and are discussed with reference to the VCCT assumption of self-similar crack growth. Numerical results applying the release methodologies are then compared with experimental results for a double cantilever beam specimen. Based on this comparison, recommendations for the future development of the degradation model are made, especially with reference to developing an approach for the collapse analysis of fuselage-representative structures

An analysis methodology for failure in postbuckling skin-stiffener interfaces

Blade-stiffened structures have the potential to produce highly efficient structures, particularly when the large strength reserves available after structural buckling, in the postbuckling range, are exploited. In experimental tests of postbuckling stiffened structures made from fibre-reinforced composites, failure typically initiates at the interface of the skin and stiffener and leads to rapid and even explosive failure. A methodology has been developed for analysing collapse in postbuckling composite structures that involves predicting the initiation of interlaminar damage in the skin-stiffener interface. A strength-based criterion is monitored in each ply using a local model of the skin-stiffener interface cross-section. For the analysis of large structures, a global analysis is first run to obtain the complete postbuckling deformation field, which is then input onto a local model using a global-local analysis technique. The coordinates of the local model can easily be moved to rapidly assess failure initiation at numerous skin-stiffener interface locations throughout the global structure. The analysis methodology is compared to experimental results for two-dimensional T-section specimens and large, fuselage-representative stiffened panels and is shown to give accurate predictions of the failure load and failure mechanisms. The use of the approach for the analysis of postbuckling composite structures has application for the design and certification of the next generation of aircraft

Degradation investigation in a postbuckling composite stiffened fuselage panel

COCOMAT is a four-year project under the European Commission 6th Framework Programme that aims to exploit the large strength reserves of composite structures through a more accurate prediction of collapse. Accordingly, one of the COCOMAT work packages involves the design of test panels with a focus on investigating the progression of composite damage mechanisms. This paper presents the collaborative results of some of the partners for this task. Different design alternatives were investigated for fuselage-representative test panels. Non-linear structural analyses were performed using MSC.Nastran and ABAQUS/Standard. Numerical predictions were also made applying a stress-based adhesive degradation model, previously implemented into a material user subroutine for ABAQUS/Standard. Following this, a fracture mechanics analysis using MSC.Nastran was performed along all interfaces between the skin and stiffeners, to examine the stiffener disbonding behaviour of each design. On the basis of the structural and fracture mechanics analyses, a design was selected as being the most suitable for the experimental investigation within COCOMAT. Though the COCOMAT panels have yet to be manufactured and tested, experimental data on the structural performance and damage mechanisms were available from a separate project for a panel identical to the selected design. This data was compared to the structural, degradation and fracture mechanics predictions made using non-linear finite element solutions, and the application of the design within the COCOMAT project was discussed

Development of a finite-element analysis methodology for the propagation of delaminations in composite structures

Analysing the collapse of skin-stiffened structures requires capturing the critical phenomenon of skin-stiffener separation, which can be considered analogous to interlaminar cracking. This paper presents the development of a numerical approach for simulating the propagation of interlaminar cracks in composite structures. A degradation methodology was introduced in MSC.Marc, which involved the modelling of a structure with shell layers connected by user-defined multiple-point constraints (MPCs). User subroutines were written that employ the virtual crack closure technique (VCCT) to determine the onset of crack growth and modify the properties of the user-defined MPCs to simulate crack propagation. Methodologies for the release of failing MPCs are presented and are discussed with reference to the VCCT assumption of self-similar crack growth. The numerical results obtained by using the release methodologies are then compared with experimental data for a double-cantilever beam specimen. Based on this comparison, recommendations for the future development of the degradation model are made, especially with reference to developing an approach for the collapse analysis of fuselage-representative structures

Compression and post-buckling damage growth and collapse analysis of flat composite stiffened panels

Experimental and numerical investigations were conducted into the damage growth and collapse behaviour of composite blade-stiffened structures. Four panel types were tested, consisting of two secondary-bonded skin-stiffener designs in both undamaged and pre-damaged configurations. The pre-damaged configurations were manufactured by replacing the skin-stiffener adhesive with a centrally located, full-width Teflon strip. All panels were loaded in compression to collapse, which was characterised by complex post-buckling deformation patterns and ply damage, particularly in the stiffener. For the pre-damaged panels, significant crack growth was seen in the skin-stiffener interface prior to collapse, which caused a reduction in load-carrying capacity. In the numerical analysis of the undamaged panels, collapse was predicted using a ply failure degradation model, and a global-local approach that monitored a strength-based criterion in the skin-stiffener interface. The pre-damaged models were analysed with ply degradation and a method for capturing interlaminar crack growth based on multi-point constraints controlled using the Virtual Crack Closure Technique. The numerical approach gave close correlation with experimental results, and allowed for an in-depth analysis of the damage growth and failure mechanisms contributing to panel collapse. The successful prediction of collapse under the combination of deep post-buckling deformations and several composite damage mechanisms has application for the next generation of composite aircraft designs

Development of a Finite Element Methodology for Modelling Mixed-Mode Delamination Growth

A critical failure mechanism for composite skin-stiffened structures in compression is separation of the skin and stiffener, which can be considered analogous to interlaminar cracking. This paper presents the extension of a numerical approach developed previously for simulating the propagation of interlaminar cracks in composite structures. The degradation methodology was implemented in MSC.Marc, and involves modelling the structures with shell layers connected by user-defined multi-point constraints (MPCs). User subroutines were written that apply the Virtual Crack Closure Technique to determine the onset of crack growth, and modify the properties of the user-defined MPCs to simulate crack propagation. In previous work, this model was applied only to specimens with Mode I crack growth, and two methods were proposed for handling the release of failing MPCs. In this paper, the model and release methods are extended to handle propagation in any crack growth mode. Numerical results applying the developed methodology are then compared with experimental results of fracture mechanics characterisation tests for Mode II and Mixed Mode I-Mode II. Based on this comparison, the capability of the model to represent delamination growth in any composite structure is demonstrated. Future work will focus on the application of the degradation model for the design and analysis of larger and more complex structures

Benchmark Finite Element Simulations of Postbuckling Composite Stiffened Panels

This paper outlines the CRC-ACS contribution to a software code benchmarking exercise as part of the European Commission Project COCOMAT investigating composite postbuckling stiffened panels. Analysis was carried out using MSC.Nastran (Nastran) solution sequences SOL 106 and SOL 600, Abaqus/Standard (Abaqus) and LS-Dyna, and compared to experimental data generated previously at the Technion, Israel and DLR, Germany. The finite element (FE) analyses generally gave very good comparison up to initial postbuckling, with excellent predictions of stiffness, and mostly accurate representations of the initial postbuckling mode shape, leading to fair omparison in deep postbuckling. Accurate modelling of boundary conditions and panel imperfections were crucial to achieve accurate results, with boundary conditions in particular presenting the most critical problem. Comparatively, SOL 106, SOL 600 and Abaqus gave almost identical results, whilst LS-Dyna produced less accurate results in postbuckling. The work in this paper will be compared to parallel FE analyses from other project partners, and conclusions will be made on the efficacy of various software codes for fuselage-representative composite structures

Parametric optimisation of composite shell structures for an aircraft Krueger flap

This paper details the conceptual design previous term optimisation next term of the configuration and previous term composite next term lay-ups used to replace the conventional honeycomb stiffened previous term structure next term of a previous term Krueger flap.next term The multiple previous term composite next term laminates selected for redesigning the lay-ups within an initial symmetrical quasi-isotropic ply configuration of [0/45/-45/90]s, had to demonstrate full orthotropic characteristics. In order to construct a numerical process to optimise the required multi-layered previous term composite shells,next term a commercial finite element code, Ansys, was used to develop a previous term parametric next term analysis file. This analysis subroutine was then integrated into an Ansys previous term Parametric next term Design Language code embedding the objective of the previous term optimisation next term process "mass minimisation" as well as all the constraints and the allowable domains of the parameters. The paper, in its conclusion, presents a comparison between the original product and the optimal design, and reviews the advantages of the future implementation of this design

Design and analysis of BWB passenger jet centre body structure

Abstract not availabl

An investigation into an advanced composites finite element explicit biphase model - Part I: Elastic Parameters

The elastic and damage parameters of the 'biphase' composite material and degradation model contained in the explicit finite element code, Pam-Shock, have been investigated in Parts I and II, respectively. The biphase analysis is a relatively new methodology aiming at accurately predicting the complex damage responses of composite structures to dynamic loading conditions. The intricacy of the damage mechanism dealt with hence calls for a broad range of elastic and damage parameters to be defined within the analysis before a solution corresponding to real case scenarios can be achieved. This investigation focuses on the unknown effects of such parameters and has been successful in identifying the significance and sensitivities that the variation of the parameters impose on the predicted outputs. It was established that the variation of some of the parameters, such as Poisson's ratio, can cause a considerable deviation from the reference run, thus making it imperative to concentrate on deriving accurate empirical values to be used against such material properties within the analysis. A brief tabulated summary demonstrates the strengths and limitations of the model in predicting the response of advanced composite structures to impact events